1. Field of the Invention
The present invention relates to an illuminator and a display device including such an illuminator, and more particularly, the present invention relates to an illuminator provided on the back side of a display panel and used as a backlight of a display device, and also relates to a display device including such an illuminator.
2. Description of the Related Art
Liquid crystal display devices are widely used in OA (office automation) equipment, car televisions, monitors for camcorders, etc., for their advantageous features such as lightweight, thin structure and small power consumption. Unlike self-luminous display devices such as CRTs, PDPs (plasma display panels) and EL (electroluminescence) devices, liquid crystal display devices use a liquid crystal display element that itself does not produce light. Therefore, in a transmission type liquid crystal display device, a planar illuminator called a “backlight” is provided on the back side of the liquid crystal display element, and the liquid crystal display element controls the amount of output light from the backlight to be transmitted therethrough in each pixel so as to display an image.
Backlights are generally classified into “direct-type” backlights including a plurality of rod-shaped light sources, such as fluorescent tubes, placed directly under a liquid crystal display element, and “edge light-type” backlights including a light source placed along an edge of a lightguide plate so that light from the light source is guided through the lightguide plate to a liquid crystal display element.
A typical structure of a direct-type backlight is schematically illustrated in FIG. 37 and FIG. 38. A backlight 40 illustrated in FIG. 37 and FIG. 38 is provided on the back side of a transmission type liquid crystal display panel 48, and includes a plurality of fluorescent tubes 41, a case 43 accommodating the fluorescent tubes 41, and an optical sheet (e.g., a diffusion sheet or a prism sheet) 45 placed between the fluorescent tubes 41 and the liquid crystal display panel 48. With the direct-type backlight 40, the brightness can easily be adjusted by adjusting the number of the fluorescent tubes 41 which define light sources, and a high brightness can be realized relatively easily. Thus, direct-type backlights are often used in large-size liquid crystal display devices.
In recent years, the length of a backlight fluorescent tube has been increasing along with the increase in the size of a liquid crystal display device. However, long fluorescent tubes have a poor anti-shock property and are difficult to handle. Moreover, as the length of a fluorescent tube increases, the operating voltage thereof increases significantly, whereby the reliability thereof decreases significantly in terms of the withstand voltage. Therefore, there is a technical limit to increasing the length of a fluorescent tube along with the increase in the size of a liquid crystal display device. Another problem is that a further increase in the size of a liquid crystal display device may necessitate the provision of a new production line for producing longer fluorescent tubes. Also, the cost and difficulty in manufacturing longer fluorescent tubes increase as the length of the fluorescent tubes increases.
Japanese Laid-Open Patent Publication No. 10-143089 discloses a backlight 40A including an array of fluorescent tubes 41′ some of which have a different length from the others, as illustrated in FIG. 39. The fluorescent tubes 41′ are arranged to be collinear along a longitudinal direction that extends along lengths of the tubes so as to form rows of tubes, and ends of the fluorescent tubes 41′ are arranged in a staggered pattern in a vertically extending direction, which is perpendicular to the longitudinal direction, over the entire array. That is, ends of two longitudinally or horizontally adjacent fluorescent tubes 41′ in one row overlap a central portion of a fluorescent tube 41′ in a vertically adjacent row of tubes in order to provide sufficient brightness at the gaps between each pair of adjacent fluorescent tubes 41′ in each row. Because this overlapping arrangement of fluorescent tubes 41′ is necessary to overcome the lack of brightness at the gaps between adjacent fluorescent tubes 41′, shorter fluorescent tubes 41′ must be used at the ends of the rows, and thus many different size fluorescent tubes are required.
The backlight 40A disclosed in Japanese Laid-Open Patent Publication No. 10-143089, which must use fluorescent tubes of different lengths as described above, has various problems due to the use of light sources of different specifications. Where light sources of different specifications are used, there are significant electrical and optical characteristics variations among different positions across the backlight, whereby it is difficult to properly control the light emission. Also, it is expensive to manufacture and assemble different size fluorescent tubes.